Thanks for the information... I'm computer-stoopid at times. I hadn't looked into which models would accept the Big Sur upgrade; I just saw that Garage Band required Big Sur, and ASSUMED that since it showed up as an upgrade that Big Sur would be available to me. So the following comedy ensued:
I watched some YouTube videos on how to clone and replace hard drives in the MacBooks... and rushed out to Best Buy and picked up a 1Tb 2.5" SSD. (I knew that I wouldn't have enough space on my current drive to do an upgrade.) Came home, cloned the drive, made sure it worked before tearing into the computer and then... where the **** is the hard drive??? Oh, wait... I've got the 15" Retina model... there is NO hard drive. Just that little SATA SSD. ****... but at least I got to blow the dust out of the fans while it was open! (And, of course, I now owned a nice 1Tb backup drive...)
Back to YouTube, making sure I'm looking at the RIGHT batch of 2012 machines this time... and then after a frustrating session with Best Buy ordered online from OWC. I got everything I needed the next day, performed the necessary surgery and -- ta-daaa! I've got an SSD with room to breathe!
And THAT'S when I went to download Big Sur... and found out that I'm no longer "in the club".
But, yes -- I do plan to keep using this computer until either it (or I) dies. I've now got room to spare, can work on videos without running out of space... and will hopefully not be shoveling out a wheelbarrow full of cash anytime soon!